The present invention relates to the field of precision aircraft landing systems. More specifically, it pertains to a landing system which calculates an aircraft's position using distance ranging calculations which are based on signals from a transponder onboard the aircraft.
Various precision aircraft landing systems have been described in the literature. Some, such as the currently used Instrument Landing System (ILS), define a conical or pyramidal shaped approach volume having its apex at the end of the runway. For these systems, the aircraft maintains a proper approach path by remaining centered within the volume along its axis. (By way of example, ILS uses a "localizer" to provide left-right guidance, a "glide-slope" to provide up-down guidance, and "marker beacons" to indicate distance from the runway.) Such landing systems virtually mandate a straight approach path, prohibiting their use at airports having natural obstacles which require a curved, stepped, or segmented approach path. Additionally, due to the complexity and associated expense of the antennas required by such landing systems, typically only a single approach path is defined per runway, preventing the use of different glide paths which is preferred when different classes of aircraft are using the runway.
Another class of precision landing system determines the aircraft's position, compares it to a desired approach path, and transmits any required correction to the aircraft. Nehama U.S. Pat. No. 3,564,543 describes such a system, as well as other systems which use symmetry and simplified mathematics to define a pyramidal approach path.
In general, the position determining system disclosed in Nehama responds to time required for radio signals to travel between the landing aircraft and three known locations on the ground to determine three respective distances. From these three distances, the aircraft's position is calculated. More specifically, the Nehama position determining systems use an interrogator and three receivers. During operation, the interrogator transmits an interrogation signal to a transponder onboard the aircraft. The transponder, in response to the interrogation signal, transmits a reply signal which is detected by the three receivers. Each receiver measures the time interval between its detection of the interrogation signal and its detection of the reply signal. From these three time intervals, the respective distances between the aircraft and the three receivers are calculated.
A small time interval, on the order of a few microseconds, transpires between the transponder's reception of the interrogation signal and its subsequent transmission of the reply signal. Unaccounted for, this interval, or "transponder reply time", can cause errors of approximately 100 meters in the calculated position of the aircraft. The Nehama patent acknowledges the existence of the transponder reply time, but does not teach a method of eliminating this factor from the measured time intervals. Instead, Nehama arranges the transmitter and receivers in a substantially vertical geometric plane transverse to the length of the runway. This arrangement projects the error in a horizontal direction along the axis of the runway, a direction considered acceptable by the Nehama patent. As a side effect, this arrangement requires the use of elevated antenna towers in the vicinity of the airport, for if all the receivers were positioned at ground level, and thus in a horizontal plane, the calculated altitude of the aircraft would contain substantial errors, which would be an impermissible situation for a precision landing system.
Meilander U.S. Pat. No. 3,665,464 discloses a system for locating aircraft. As with the Nehama system, it times intervals between detection of interrogation and transponder reply signals. Meilander also acknowledges the transponder reply time, and accounts for it by subtracting its specified value from the measured time intervals. However, even a specification on the transponder reply time will allow for measurable amounts of "jitter", that is, variance from the mean, in the transponder reply time. This, again, results in considerable imprecision in the determination of the landing aircraft's position.
What is needed, therefore, is a precision aircraft landing system which determines on a real-time basis the location of an aircraft by measuring time intervals between detection of interrogation and transponder reply signals at a plurality of predetermined locations, and avoids imprecision by negating the effect of the transponder reply time.